Scientists
are using gene therapy to restore vision in blind dogs. A genetic defect
in affected dogs causes the retina to degenerate, leading to permanent
blindness.

A montage of images taken of the retina of the
blind dog. The white arrow marks the site where the virus carrying
the RPE65 gene was injected.Courtesy Jean Bennett

Jean Bennett, of the University of Pennsylvania, and colleagues studied
Briards that have a mutation in both copies of their RPE65 gene.
Without a healthy copy of RPE65, the dogs' retinal cells do not
produce rhodopsin molecules, which detect light photons and translate
them into electrical signals that are sent to the brain.

"We have rescued defective retinal cells that can now translate
light into electrical impulses and send signals to, and communicate with,
the brain," emphasizes Bennett. "This is not just a small local
genetic change that we have made to a few cells in the retina."

Bennett's team used a virus to carry a healthy copy of RPE65 into
a region of the retina containing light-sensing cells. The virus infected
the retinal cells and released its therapeutic cargo, RPE65. Cells
with a healthy copy of RPE65 produced rhodopsin, allowing the dogs
to see.

The canine disease is similar to that seen in infants with Leber congenital
amaurosis (LCA). "LCA is the most devastating form of retinal degeneration,"
says Bennett. "Children diagnosed with this form of blindness are
sent to learn Braille, work with guide dogs and use a cane." The
dogs may provide a useful model for studying retinal diseases in humans
and developing therapies.

To measure the effects of gene therapy on the dogs' behavior, the animals
were placed in a dimly lit obstacle course. "An untreated animal
looks like a ball-bearing shot out of a pinball machine," explains
Bennett. The animals are unable to avoid the obstacles and bump into things
on the left and right. Animals that received therapeutic genes in the
right eye avoided objects directly in front of them and on their right.
They only bumped into objects on the left side.

"The dogs seem to prefer the treated eye and look in that direction,"
says Bennett. "They must be both thrilled and puzzled." She
and her colleagues are confident the treated dogs are able to see, but
they are not sure how well. The researchers will continue to monitor the
dogs' visual acuity.

To test whether the modified retinal cells were transmitting signals
to the brain, Bennett's team measured the electrical activity as the dogs
were exposed to blue light. In healthy dogs the retinal cells showed increasing
electrical activity, as the light became more intense. Treated dogs that
had received a healthy copy of RPE65 showed a similar response.
In untreated animals the light evoked barely any electrical activity.

Bennett's team also measured the diameter of the pupil after exposure
to light. In healthy and treated animals the pupil diameter constricted
in response to the light. In untreated affected animals the pupil diameter
remained almost constant.

"This really shows us that we have restored communication between
the retinal cells and the brain," says Bennett. "The cells were
able to detect light and send a signal to the brain. And the brain responded
telling the pupil to constrict."

The healthy RPE65 gene was present and active in retinal cells
nine months after treatment. The dogs' ability to see appears to have
remained constant.

Bennett and her team are interested in the potential of gene therapy
to treat LCA in humans. This rare disease is usually diagnosed in infancy
when parents notice that their child's eyes wander constantly. The roving
eye movement occurs because the child is unable to focus on specific objects.
Eventually the light-sensitive cells in the retina degenerate, eliminating
the opportunity for repair.

The researchers are conducting tests to ensure that the gene therapy
is not toxic. Clinical trials could begin in as early as two years.